Ever since the demonstration of low operating voltages by Tang et al. 1987 (C. W. Tang et al., Appl. Phys. Lett. 51(12), 913 (1987)), organic light-emitting diodes have been promising candidates for the realization of large-area displays and other uses, such as, e.g., lighting elements. They consist of a sequence of thin (typically 1 nm to 1 μm) layers of organic materials, which are preferably vapor-deposited in vacuum in the form of small molecules, whereby so-called OLEDs are produced, or are spun on from a solution, pressed or deposited in another suitable form (polymers), whereby so-called PLEDs are produced. By injecting charge carriers (electrons from one side, holes from the other side) from the contacts into the organic layers situated therebetween as a result of an externally applied voltage, the subsequent formation of excitons (electron-hole pairs) in an active zone and radiant recombination of these excitons, light is produced and emitted by the light-emitting diode.
Usually, organic light-emitting diodes in the form of PLEDs are based on the following layer structure:                1. Substrate (transparent, e.g. glass)        2. Anode (transparent, usually indium tin oxide (ITO)        3. Hole-transporting layer or hole-injecting layer (usually PEDOT:PSS or PANI=polyaniline with admixtures such as PSS; PEDOT=polyethylenedioxythiophene, PSS=polystyrene sulfonate)        4. Active polymer (emits light)        5. Cathode (usually a metal having a low work function, such as barium, calcium)        
The polymeric layers, i.e., the hole transporting or hole-injecting layer and the active polymer are prepared from a liquid solution (in water or solvents). The contacts (anode, cathode) are typically produced by vacuum processes.
The advantages of this structure for applications such as displays is the variety of processes available for the preparation of the polymeric layers, including processes permitting simple lateral structuring of the PLEDs, namely ink-jet pressing. In this process, the different polymers of three colors are pressed on at previously prepared sites, whereby adjacent regions of different emission color are obtained.
The drawback consists, among other things, in the fact that not more than two different polymeric layers can be rationally applied, since the solvents of the polymers must be selected in such a way that they do not mutually affect each other, and, in other words, they do not attack the substrate material. This means that the emitting polymer must also be simultaneously well suited for both electron transport and electron injection from the cathode, a requirement which represents a serious limitation in the selection of material and structure optimization.
On top of this, the sequence of the structure for a given material system can be changed only with difficulty; thus, as in the above case, one must start with the anode. This is disadvantageous particularly for the integration of the PLEDs on active-matrix display substrates with n-channel transistors as a switch component. The use of transparent cover contacts (also as cathode) is just as difficult, since they are usually prepared by a sputter process (e.g., ITO). However, this destroys organic materials. Since the topmost layer in a PLED is an emitting layer, the efficiency of light production of the organic light-emitting diode is thereby reduced. An improvement of the stability against sputter damages can be obtained by introducing a layer vapor-deposited in vacuum, consisting of small molecules. However, even in this case the electron injection from the cathode represents a problem. A further drawback of the above structure is that an efficient electron injection can be achieved only with very unstable contact materials such as barium or calcium. These materials, however, are attacked by oxygen and water.
Organic light-emitting diodes in the form of OLEDs are built up of small molecules that are vapor-deposited in vacuo. If the small molecules which are to form the layers of the OLEDs are small enough, they can usually be deposited by a thermal process without decomposition. To this end the molecules are vaporized in vacuo (because of the long free path).
To improve the injection from the contacts into the organic layer and increase the conductivity of the transporting layers, the transporting layers may be doped by mixed evaporation with organic or inorganic dopants which are acceptors (for hole doping) or donors (for electron doping). In this case, the dopants must not, at the beginning of the evaporation process, be present in their final form, as long as the alternatively used precursor material forms the dopant during the evaporation process (which can be modified as well, e.g., through the use of electron rays). The mixed layers are typically prepared by mixed (co)vaporization.
In addition to the doped transporting layers it is necessary to then introduce intrinsic (i.e., not doped) intermediate layers having specified energetic properties (Patent DE 100 58 578, M. Pfeiffer et al., “Light-emitting component comprising organic layers”, filed on Nov. 20, 2000; X. Zhou et al., Appl. Phys. Lett. 78, 410 (2001)).
In that case, the structure of the OLED is a p-i-n heterostructure:                1. Carrier, substrate,        2. Electrode, hole-injecting (anode=positive pole), preferably transparent,        3. p-doped hole-injecting and transporting layer,        4. Thinner hole-side blocking layer of a material whose band positions match the band positions of the layers surrounding it,        5. Light-emitting layer,        6. Electron-side blocking layer (typically thinner than the layer mentioned below) of a material whose band positions match the band positions of the layers surrounding it,        7. n-doped electron-injecting and transporting layer.        8. Electrode, usually a metal having a low energy function, electron-injecting (cathode=negative pole).        
Advantages of this structure are the separate optimizability of the properties of the individual layers, the large adjustable distance between the emitter layer and the contacts, the very good injection of the charge carriers into the organic layers, and the low thickness of the layers whose conductivity is not very good (4; 5; 6). In this way, very low operating voltages (<2.6 V for a light density of 100cd/m2) at a simultaneously high light production efficiency can be achieved, as described in J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, Sh. Liu and K. Leo in Appl. Phys. Lett. 80, 139-141 (2002): “Low-voltage organic electroluminescent devices using pin structures.” As shown in DE 101 35 513.0 and in X. Q. Zhou et al., Appl. Phys. Lett. 81, 922 (2002), this structure can, in addition, be easily inverted and top-emitting and fully transparent OLEDs can be realized, as described in DE 102 15 210.1.
The drawback of this structure is that lateral structuring of the OLED structure for the build-up of different-color pixels in one display can only be carried out through shadow masks. This process has limitations with regard to the smallest achievable pixel sizes (<50 μm subpixels). In a manufacture, the shadow mask process is a relatively expensive process. To be sure, the ink-jet process cannot be used in the case of small molecules, due to their insolubility.
U.S. 2003/020073 A1 describes the use of vapor-deposited blocking layers and electron-transporting layers on a polymeric hole-transporting layer. In this arrangement, the possibility exists of structuring the polymeric layer laterally, in order to produce a full color display. However, with this arrangement, the injection of charge carriers (in this case, electrons from the cathode into the molecular electron-transporting layer) is problematical, which increases the operating voltage of the hybrid polymer-small molecule OLED.
Hence, it is the object of the invention to increase the flexibility of construction of a light-emitting component and the injection of charge carriers into the organic layers, while maintaining a good structurability.
This object is achieved from the arrangement point of view by arranging at least one polymer layer and two molecular layers, and, when the cover contact is a cathode, the layer adjacent to the cover contact is formed as an electron-transporting molecular layer and is doped with an organic or inorganic dopant, the n-type dopant containing a principal organic substance and a donor-type doping substance, and the molecular weight of the dopant is greater than 200 g/mole; or, when the cover contact is an anode, the layer adjacent to the cover contact is formed as a p-doped hole-transporting molecular layer and is doped with an organic or inorganic acceptor, the dopant containing a principal organic substance and an acceptor-type doping substance, and the molecular weight of the dopant is greater then 200 g/mole. Through the incorporation of molecular layers it is possible to achieve a considerably greater flexibility in the layer composite, while the simultaneous presence of polymer layers assures easier structurability without the special use of shadow masks.
The dopant should consist of an organic, inorganic or organometallic molecule, which has a molecular weight of more than 200 g/mole, preferably more than 400 g/mole. What matters here is that the dopant active in the layer have this molecular weight. For example, Cs2CO3 (cesium carbonate, molecular weight about 324 g/mole) is unsuitable, within the meaning of the invention, as donor for n-doping of the electron-transporting layer. Cs2CO3 as such is a comparatively stable compound which is no longer in a position to transfer one or more electrons to another molecule (the matrix material). To be sure, molecular Cs can be liberated in a vaporization process above 615° C. (decomposition temperature), and this Cs would be able, as dopant, to transfer an electron to the matrix material. However, the molecular weight of Cs is about 132 g/mole. Cesium, as dopant, has the disadvantage that, as a relatively small molecule or atom, it cannot be incorporated in the matrix layer in a diffusion-stable manner, and has negative effects on the service life of the organic light-emitting component. The same applies in the case of p-doping of the hole-transporting layer with a strong acceptor (in the case of an inverted POLED construction).
The two molecular vapor-deposited layers are the non-doped intermediate layer (reference numeral 5 in the embodiment described below) and the doped transporting layer. Since the energy barrier of the charge-carrier injection from the doped transporting layer into the polymeric emitting layer is too large for common emitter polymers such as polyphenylenevinylene, PPV (in the case of the traditionally known layer structure with polymeric hole-transporting layer on a substrate, the barrier for the injection of electrons), a non-doped intermediate layer must be inserted which is considerably thinner than the doped transport layer and whose LUMO energy level (LUMO: lowest unoccupied molecular orbital), and, to be sure, in case of the hole-transporting layer, the HOMO energy level (HOMO; highest occupied molecular orbital) must be between the doped transporting layer and the emitter polymer layer. This has the consequence, on the one hand, that charge carriers can be more effectively injected into the emitter polymer layer, and on the other hand, that nonradiant recombination processes also occur at the interface between the emitter polymer layer and the doped transporting layer, these usually taking place almost inevitably at high energy barriers.
From the process point of view, the object of the invention is achieved by arranging at least one of the layers as a polymer layer and vapor-depositing at least one of the layers as a molecular layer, said molecular layer being doped.
Advantageously, the doping of the molecular layer is carried out in a vacuum as a mixed vapor deposition from two separately controlled sources.
The deposition of the polymer layers can be carried out in a very precise manner by using simple means. This structuring serves, at the same time, for structuring the later light-emitting component, without the necessity of expensive structuring steps or structuring means. By contrast, the deposition of molecular layers prevents a situation where, as a result of the presence of usually only two disjunct solvents, the modification of polymer layers will be very limited and increase the possibility of the build-up of the most varied layer combinations.
Below, the invention will be explained in greater detail on the basis of one embodiment.